The human enzyme glucose-6-phosphate dehydrogenase (“G6PD”) performs a critical function in human biochemistry. It is part of the oxidative pentose pathway, wherein it functions to minimize oxidative attacks of free radicals upon cells by providing reducing equivalents-i.e., G6PD converts glucose-6-phosphate to 6-phosphoglutonate, thereby liberating a proton that reduces nicotinamide adenine dinucleotide phosphate, NAPD, to NAPDH. The NAPDH initiates a series of downstream reactions that ultimately reduce the free radical oxidizing agents and render many of them ineffective in normal human biochemistry.
G6PD is present in all human cells, but it is in higher concentration in red blood cells which, in one of their primary functions, act as oxygen transport vehicles and are hence particularly susceptible to oxidative attack. The efficiency of the G6PD system is remarkably high as reflected by the fact that during normal activity less than 1% of its capacity is utilized in combating and preventing undesirable oxidative effects. However, when strong oxidizing agents, such as members of the quinine class of anti-malarial drugs must be introduced to humans, the need for rapid production of reducing agents is greatly increased.
Several mutations of the gene which encodes for G6PD are known which decrease the efficiency of the enzymes in the biochemistry of individuals processing such a mutation in both halves of their genome, causing the quantity of their G6PD to remain at the same level as in people with a normal gene, but also causing their G6PD to show greatly reduced specific activity. In these individuals, administration of strong oxidizing agents such as members of the class of quinine-type anti-malarials, may cause severe clinical complications, such as hemolytic anemia, because the low specific activity of their G6DP does not enable the production of sufficient reducing agents to prevent rapid unwanted oxidative effects on their red blood cells. In areas where malarial infections are common and at times even epidemic, a need therefore exists for a rapid efficient test that will readily distinguish persons having G6PD of low specific activity from persons whose G6PD activity is normal and will enable medical personnel to ensure that (1) the quinine antimalarials are prescribed only for individuals with normal or better G6PD specific activity and (2) persons with lower than normal G6PD activity are medicated with an alternative type of anti-malarial drugs.
Heretofore, assays that involve enzyme activity, in any context, have most usually been conducted in “wet chemistry” formats which require trained laboratory personnel to prepare for and perform them. The reagents for such assays must either be made fresh from dry components or be reconstituted from commercially available dried formulations. Wet reagents are less stable than dried ingredients or dried formulations and, to the extent they must be stored, more stringent, carefully monitored storage conditions, including special handling techniques to prevent contamination, are required. Those assays also require instruments such as spectrophotometers, fluorimeters or other such instrumental equipment to read the endpoint results of the assay. Such assays are not practical for use in doctor's offices, hospitals and nursing home facilities, under epidemic conditions, or for home or field use.
Automated clinical chemistry analyzer systems are in industrial use which perform dry chemistry formatted assays wherein the presence, absence, concentration or specific activity of a substance present in or absent from a sample is determined. Such a substance is, for purpose of this application, referred to as the “analyte” and it may be an enzyme per se (as in the G6PD assay hereinafter described in detail) or a substance necessary to the elicitation of a specific enzyme activity. Examples of automated clinical chemistry analyzer systems are the Johnson and Johnson Vitros™ and the Roche Cobas™ systems. These and similar automated systems are not subject, when performing as designed, to the preparation skill requirements and shelf life problems associated with humanly performed dry chemistry assay work. Because programmed robots perform the manipulative tasks, the need for intensively trained humans is likewise avoided. The systems, however, require on board reader instrumentation and they are necessarily too large, too complicated and generally too burdened with infrastructure requirements to be practical for use in doctor's offices or homes and in many hospitals, clinics and like places. Clearly, they have too many technical requirements for field use.
There are available, as well, a very few non-instrument based dry-chemistry assays, such as the Orasure QED™ assay for alcohol which is based upon use of the alcohol dehydrogenase enzyme to determine alcohol content of saliva in the field. This and other known assays of this genre have heretofore been limited to determinations that can be made on samples that are free of substances that may obscure, inhibit or in some other manner intrinsically interfere with and render imprecise determinations that are dependent upon some aspect of enzymatic action or content.
An example of an enzymatic assay that operates on samples containing visually obscuring substances and uses antibody capture zones to select for enzyme analytes is shown in U.S. Pat. No. 5,506,114. This system requires wash steps to remove the visually obscuring substances and is sufficiently cumbersome to perform that it is impractical for field use or use in doctor's offices, homes, most clinics and many hospitals and the like.